1. Field of the Invention
The present invention generally relates to components formed of composite materials, such as a ceramic matrix composite (CMC). More particularly, this invention relates to CMC components that require securement with metal fasteners to adjacent metal components in high temperature applications, such as gas turbine engines.
2. Description of the Related Art
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. While significant advances in high temperature capabilities have been achieved through the formulation of iron, nickel and cobalt-base superalloys, the high temperature properties of these metal alloys alone are often insufficient to withstand long exposures to operating temperatures within the turbine, combustor and augmentor sections of some high-performance gas turbine engines. As a result, internal cooling of components such as combustion liners, blades (buckets) and nozzles (vanes) is often employed, alone or in combination with thermal barrier coating (TBC) systems on their exterior surfaces. Alternative materials have also been investigated, such as ceramic matrix composite (CMC) materials whose high temperature capabilities are able to significantly reduce cooling air requirements. CMC materials, particularly continuous fiber-reinforced CMC materials, are currently being considered for shrouds, combustor liners, nozzles, and other high-temperature components of gas turbine engines.
The use of CMC""s in gas turbine engines has created a need for high temperature attachments to interface with adjacent metal support structures and components. The large difference in thermal expansion rates between metals and CMC""s creates unique attachment design problems, especially when metal fasteners are used. Metal fasteners, such as pins, bolts, etc., are desirable because of their high fracture toughness and their compatibility with adjacent metal components. Because of the significant difference in coefficients of thermal expansion (CTE) of a CMC component, its metal support structure, and the one or more metal fasteners used to secure the component to the support structure, clearance must be provided to permit the fastener and support structure to expand and contract relative to the CMC component during thermal excursions. However, CMC materials are harder than most metals, and therefore tend to wear into metal fasteners during the long periods of component vibration inherent in gas turbine operation. FIG. 1 represents such a situation, in which a CMC component 10 is secured to a metal structure 12 with a metal pin 14, and shows the shank 16 of the pin 14 as being severely worn as a result of vibration or other relative movement between the component 10 and structure 12. In FIG. 1, the diametric clearance between the pin 14 and component 10 is exaggerated for purposes of illustration. Wear is shown as having occurred on the shank 16 of the pin 14 as a result of the pin 14 being formed of a softer material than the CMC material that forms the component 10.
Problems can arise when a transient thermal condition occurs, causing relative motion between the pin 14 and the CMC component 10 as a result of the abrupt temperature change and the significant CTE mismatch between the component 10 and pin 14. For example, FIG. 2 represents the perimeter 18 of the through-hole in the CMC component 10 of FIG. 1 as being interlocked with the worn shank 16 of the pin 14 as a result of a transient thermal condition. The resulting restraint of the CMC component 10 can cause a sudden overload of the component 10, or localized damage to the component 10 that can expose damaged laminae to environmental decay, such as by oxygen embrittlement.
In view of the above, it would be desirable to avoid or at least minimize the damage that CMC components may sustain when secured with metal fasteners to a metal structure and subjected to differential thermal expansion and contraction under transient thermal conditions.
The present invention provides an assembly that includes a CMC article, and a method of forming the assembly, so as to reduce the likelihood during a transient thermal condition of the CMC article becoming interlocked with the worn shank of a fastener used to secure the article to its support structure.
A CMC article to which this invention generally applies has oppositely-disposed first and second surfaces, a hole through the article and intersecting the first and second surfaces so as to define oppositely-disposed first and second openings at the first and second surfaces, respectively. According to the invention, the CMC article is fabricated to have continuous chamfers along the entirety of the first and second openings. The assembly in which the CMC article is installed includes a support structure adjacent the article, and a fastener received in the hole of the CMC article and securing the CMC article to the support structure. The location of the chamfers at the perimeter of each opening eliminates a relatively sharp edge that could interlock with the worn shank of the fastener during a thermal excursion.
In view of the above, it can be appreciated that the CMC article of this invention is able to avoid or at least minimize the potential damage that might otherwise occur as a result of differential thermal expansion and contraction of the article relative to its support structure during transient thermal conditions. The assembly method by which this benefit is obtained simply involves fabricating the CMC article to have the dual chamfered openings, and then mounting the article to the support structure by positioning the article adjacent the structure and then securing the article with a fastener that is received in the chamfered hole. By accommodating the inevitable wear that occurs between a relatively hard CMC article and a softer (e.g., metal) fastener used to secure the article, the article is less likely to become unduly restrained by the fastener during a transient thermal condition.
Other objects and advantages of this invention will be better appreciated from the following detailed description.